Ch2part3

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Chapter 2:

C++ FUNDAMENTALS(Part 3)

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DCS5088 :: Chapter 2 (part 3) 2

Objectives

At the end of this lecture, students should

be able to :

Create functions and implement themcall by value

call by reference (pointers)

call by reference (reference arguments) Understand variables scope

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2.15 Functions

Modules

The C++ library- a rich collection of functions and classes

Programmer defined functions, classes

A function invoked by a function call.

The function call specifies the function nameand

provides informationthat the call function needs to do

its job.

Funct ionsallow the programmer to modular izea

program.

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2.15.1 Analogy

Boss to worker analogy

A boss (the calling function or caller) asks a

worker (the called function) to perform a taskand return (i.e., report back) the results

when the task is done.

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2.16 Math Library Functions

Perform common mathematical calculations Include the header file

Functions called by writing

functionName (argument);

or functionName(argument1, argument2, );

Example

cout

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2.17 Functions: 3 things to include in a

program

Function definition / called function

Function prototype

Function call / invocation or callingfunction

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#include

using namespace std;// function prototypes here

.

void main(){ .

//function call

..

}

//function definition

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2.17.1 Function Definition

Below is the format:Return-value-type function-name (parameter-list)

{

declarations and statements}

Parameter list may be empty or have 1 or

more variablesVariables in the parameter list have a local scope.

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Function Def. Example 1

Create a function that accepts 2 integers and return

the total as an integer value

intfunc1(int n1, int n2){ int total;

total = n1 + n2;

return total;

}

How many

localvariables are

there?

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Create a function that accepts 2 integers and

displays the total result

voidfunc2(int n1, int n2)

{ int total;

total = n1 + n2;

cout

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Create a function that gets an integer number from

user and returns the number

intfunc3( ){ int num;

cin>>num;

return num;

}

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Create a function that gets an integer number from

user and displays the square of the number

void func4( ){ int num, result;

cin>>num;

result = num * num;

cout

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2.17.2 Function prototypes

A function prototypetells the compiler the

the nameof a funct ion,

the typeof the datareturned,

the numberof parameters,

the typeof those parameters,

the orderin which the parameterof those types

are expected.

The compiler use function prototype to validate

functioncalls.

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Function Prototype. Example 1

Given the function definition:

intfunc1(int n1, int n2)

{ int total;total = n1 + n2;

return total;

}

So, the function prototype:

int func1(int, int);

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Function Prototype. Example 2

Given the function definition:intfunc3( )

{ int num;

cin>>num;

return num;

}

So, the function prototype:int func3();

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2.17.3 Function call

Function that returns a value to the caller.

need to assign the value returned by the function to avariable on the left

Example:

val = func1(a, b);

n1 = calculate (100, 2);

Function that does not return any value to the caller

simple

Example:

func4( );

display(199.59);

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2.18 Methods of Data accessibility for

functions

Call by value

Call by reference using pointers

Call by reference using referenceparameters

Global variables

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Call by Value

When the data is passed to a function,

the value of the variable is copied into the

input variable parameters. Called function has own copies of data

passed from calling function

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Full program Example 1

Create a function that accepts 2

arguments from the main() and returns

the total to main() In the main, get user inputs for 2 integers

and call the function to return the total.

Display the total

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DCS5088 :: Chapter 2 (part 3)20

#includeusing namespace std;int calc(int, int);

int main(){ int n1,n2, total;

coutn1>>n2;

total = calc(n1, n2);

cout

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Output:

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Full program Example 2

Create a function that accepts 2

arguments from the main() and displays

the total result In the main, get user inputs for 2 integers

and call the function to show the total.

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DCS5088 :: Chapter 2 (part 3)23

#includeusing namespace std;void calc(int, int);

int main(){ int n1,n2;

coutn1>>n2;

calc(n1, n2);

return 0;}

void calc(int a, int b){

int total;total = a + b;cout

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Output:

Still same as example 1 but differentimplementation

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2.18 Methods of Data accessibility for

functions

Call by value

Call by reference using pointers

Call by reference using referenceparameters

Global variables

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Call by Reference (using pointers)

The address of the data variable is passed

from the calling function(from main) to a called

function. The address is copied into a pointer

variable that belongs to the called function.

Called function does not have its own local

copy of the variable.

The indirection operator(*) is used with thepointer to access the calling functions data.

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Example 1

Create a function that accepts 3 arguments

from the main() through pointers. The first two

pointers refers to the integers from main and

the third pointer refers to the result variable.The function will set the result variable to the

addition of the 2 integers at main().

In the main, get user inputs for 2 integers and

call the function. After that show the total.

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DCS5088 :: Chapter 2 (part 3)

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#includeusing namespace std;void calc(int*, int*, int*);

int main(){ int n1, n2, total;coutn1>>n2;

calc(&n1,&n2, &total);cout

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Call by Reference (using

reference arguments) Another technique for passing the variables address to

a function

A reference parameter is an address. Reference

parameters employ the reference operator (&) in thefunction declaration. The reference variables in called

function are implicit pointers

The variable name is used in the call statement(from

main) but actually the address of the variable is passed

to the called functions parameters.

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Example 1

Create a function that accepts 3 arguments

from the main() through reference arguments.

The first two reference argument refers to the

integers from main and the third referencerefers to the result variable. The function will

set the result variable to the addition of the 2

integers at main().

In the main, get user inputs for 2 integers and

call the function. After that show the total.

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DCS5088 :: Chapter 2 (part 3)

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#includeusing namespace std;void calc(int&, int&, int&);

int main(){ int n1, n2, total;coutn1>>n2;

calc(n1,n2,total);cout

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2.19 Variable Scope

WHERE you declare a variable

Local

A local variable is a variable that is declared

within a function, or within any block of code.

Global

A variable declared outside of any function

can be used in any function

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2.20 Static Variable

A local variable that retains its value in a

function that can be called a number of

times till the entire program terminates.

They are still known only in the function in

which they are declared,

Retain their values when the function

returns to its caller.

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